Monitor Mouse Event for any arbitrary application on macOS in Swift

gistfile1.swift

let callback: CGEventTapCallBack =  { (tapProxy, eventType, event, refcon) -> Unmanaged<CGEvent>? in
    debugPrint("we are monitoring the mouse event here")
    return nil
}

let eventMask = (1 << CGEventType.leftMouseDown.rawValue) | (1 << CGEventType.leftMouseUp.rawValue)
let machPort = CGEvent.tapCreateForPid(pid: 40529, place: .tailAppendEventTap, options: .defaultTap, eventsOfInterest: CGEventMask(eventMask), callback: callback, userInfo: nil)!


let runLoopSource = CFMachPortCreateRunLoopSource(kCFAllocatorDefault, machPort, 0)
CFRunLoopAddSource(CFRunLoopGetCurrent(), runLoopSource, .commonModes)
CGEvent.tapEnable(tap: machPort, enable: true)
CFRunLoopRun()

Is there any way to use this code to prevent events from being forwarded to the application? I am trying to prevent my cursor from being replaced whenever I hover over the dock. Even tho I return nil within the callback, the dock still receives the events.